In the gas turbine engine industry, there continues to be a need for improved corrosion- and oxidation-resistant protective coatings for nickel-base and cobalt-base superalloy components, such as blades and vanes, operating in the turbine section of the gas turbine engine. The use of stronger superalloys that often have lower hot corrosion resistance, the desire to use lower grade fuels, the demand for longer life components that will increase the time between overhaul and the higher operating temperatures that exist or are proposed for updated derivative or new gas turbine engines underscore this continued need.
Diffused aluminide coatings have been used to protect superalloy components in the turbine section of gas turbine engines. In a typical example, an aluminide coating is formed by electrophoretically applying an aluminum-based powder to a superalloy substrate and heating to diffuse the aluminum into the substrate. Chromium is used to control the aluminum activity of the powder. Such coatings may include chromium or manganese to increase the hot corrosion/oxidation resistance thereof.
It is known to improve the hot corrosion- and oxidation resistance of simple diffused aluminide coatings by incorporating a noble metal, especially platinum, therein. Such platinum-enriched diffused aluminide coatings are now applied commercially to superalloy components by first electroplating a thin film of platinum onto a carefully cleaned superalloy substrate, applying an activated aluminum-bearing coating on the electroplated platinum coating and then heating the coated substrate at a temperature and for a time sufficient to form the platinum-enriched diffused aluminide coating on the superalloy substrate. Optionally, the platinum may be diffused into the substrate either prior to or after the application of the aluminum. e, g., "Platinum Modified Aluminides-Present Status," J. S. Smith, D. H. Boone (1990). The platinum forms an aluminide of PtAl.sub.2 and remains concentrated toward the outer surface regions of the coating.
It is also known to improve the hot corrosion/oxidation resistance of diffused aluminide coatings by alloying the coating with silicon. Particularly, U.S. Pat. No. 5,057,196 to Creech et al. discloses a platinum-silicon coating which is electrophoretically deposited on a nickel or cobalt superalloy substrate. The deposited powder is heated to form a transient liquid phase on the substrate and initiate diffusion of Pt and Si into the substrate. An aluminum-chromium powder is then electrophoretically deposited on the Pt--Si enriched substrate and diffusion heat treated to form a corrosion- and oxidation-resistant Pt--Si enriched diffused aluminide coating on the substrate. The presence of both Pt and Si in the aluminide coating unexpectedly improves coating ductility as compared to a Pt-enriched diffused aluminide coating without Si on the same substrate material.
As further background, it is known that the ability to electrophoretically coat a conductive substrate depends on an electrophoretically active agent such as a zein/cobalt nitrate complex in the bath to produce a migration of the particles toward the substrate. In order to transfer coating particles from the bath suspension to the substrate, the zein complex must wet the coating particles. Because of this wetting, almost any particle compound (elemental powders, metal alloys, or ceramic compounds) can be electrophoretically deposited.
A typical bath composition contains 20-30 grams/liter of solids and 2-3 grams/liter of the soluble zein complex. Typically, the coating is deposited by using a direct current at a current density of 1-2 mA/cm.sup.2 and a voltage necessary to drive the current.
The deposition of the green coat becomes self-leveling as time passes because once the coating thickness reaches a certain threshold, the deposition rate approaches zero. Provided this green coat thickness produces the desired diffused coating thickness for a particular substrate/coating combination, the final coating thickness is diffusion controlled. Coating systems with diffusion control are ideally suited for complex part geometries.
In cases where the as-deposited coating weight is beyond the desired mass per unit area, a way to control the final coating thickness is necessary. The simplest method is by controlling the weight applied by shortening the deposition cycle. In this method, the diffused coating thickness is determined by the amount of material deposited on the part. This method is not always satisfactory for coating complex shape parts though, since areas with locally high current densities end up with higher local green coat weights, while areas with locally lower current density areas end up with lower green coat weights. These uneven green coat weights produce an uneven diffused coating thickness.
Other possible variables that may afford improved uniformity of the applied green coat include: 1) anode shape, 2) anode to part distance, and 3) anode/cathode area ratio. However, if a thin uniform green coat is desired, experience has shown that the use of these factors is limited. The time required to produce a thin coating is not long enough for these parameters to be effective.
As an alternative to these prior art methods, the present invention provides a method for controlling coating thickness that relies on the diffusional flow of coating material. In this method, a sufficiently high quantity of coating is applied and the diffusion time and temperature determine the final coating thickness, with the remainder of the undiffused deposit being removed by a simple grit blast. For simple aluminide coatings (e.g., U.S. Pat. No. 3,748,110) the composition of the coating is such that the final diffused coating thickness is nearly independent of the applied coating thickness and diffusional control works very well. For parts with complex geometries, the areas of locally higher current density as well as those with lower current density have nearly the same diffused coating thickness provided a threshold green coat weight of about 15 mg/cm.sup.2 is applied. Diffusion limited coating thickness is therefore a preferred method of controlling the final coating thickness because diffusion conditions are more easily controlled than green coat weight for complex shapes.
Accordingly, the present invention adapts current patent technology (e.g., the technology disclosed in U.S. Pat. No. 5,057,196) and modifies it to make the platinum-silicon (Pt--Si) application step one of diffusional control rather than of green coat weight control.